1. Field of the Invention
The present invention relates to optical modulation apparatuses for performing optical modulation.
2. Description of the Related Art
Optical devices widely used in optical communication include optical modulators. In some optical modulators, an optical waveguide is formed on a substrate, and an input electric signal is converted to an optical signal by applying the voltage to the optical waveguide to change the amount of light absorption on the optical waveguide, thereby modulating the light intensity (external modulation).
FIG. 11 shows the structure of a conventional optical modulator 5. The optical modulator 5 includes a light source 50, an external modulation unit 51, a coupler 52, a photodiode (PD) 53, a bias control unit 54, and a drive unit 55.
The external modulation unit 51 has a Mach-Zehnder interferometer formed by an optical waveguide on a crystal substrate using a material having the electrooptic effect, which means that the refractive index changes with the application of an electric field, such as lithium niobate (LiNbO3: LN). The Mach-Zehnder interferometer splits input light into two branches and combines them later.
The optical waveguide is split into two parallel waveguides 51a-1 and 51a-2. In the figure, a signal electrode 5b is formed in a vicinity of the parallel waveguide 51a-1, and ground electrodes 5c are formed on both sides of the signal electrode 5b. (The figure shows a single-electrode structure on a Z-cut substrate.)
The bias control unit 54 contains a low-frequency oscillator, adds an oscillated low-frequency signal to an internally-generated DC voltage, and outputs a low-frequency signal biased with the DC voltage.
The drive unit 55 performs amplitude modulation of the low-frequency signal, based on an input data signal of ‘0’ or ‘1’, generates a low-frequency superimposed signal by adding the low-frequency signal to the input data signal, and inputs the superimposed signal to the signal electrode 5b of the external modulation unit 51. The external modulation unit 51 outputs an optical signal intensity-modulated by the low-frequency superimposed signal.
The PD 53 converts light output from the external modulation unit 51 and split by the coupler 52 into an electric signal and sends the signal to the bias control unit 54. The bias control unit 54 extracts the frequency component of a returned low-frequency signal included in the received electric signal, compares the phase of the frequency component of the internally oscillated low-frequency signal and the extracted frequency component of the low-frequency signal, detects a DC voltage that optimizes the operating point of the external modulation unit 51, and biases the low-frequency signal with the DC voltage.
The drive unit 55 generates a low-frequency superimposed signal by adding the optimally biased low-frequency signal to the input data signal and drives the external modulation unit 51 with the low-frequency superimposed signal.
A technique for controlling the amplitude of the drive signal of an LN modulator in variable increments (Japanese Unexamined Patent Application Publication No. 2004-247968 (paragraphs [0018] to [0024], FIG. 1) has been present conventionally.
In the external modulation unit 51, the operating point of optical modulation varies with temperature or time. Therefore, the optical signal output from the external modulation unit 51 and separated by the coupler 52 is converted to an electric signal by the PD 53; a low-frequency superimposed signal (pilot superimposed signal) is generated on the basis of the results of detection of the low-frequency signal (pilot tone) included in the electric signal; and automatic bias control (ABC) specified in the external modulation unit 51 is performed to optimize the bias voltage, as described above.
If the frequency component of the pilot tone appears in the electric signal, the bias voltage does not provide the optimum operating point. If the frequency component of the pilot tone does not appear in the electric signal, the bias voltage has been optimized. With this feedback control, the external modulation unit 51 is always controlled to operate at a constant operating point.
FIG. 12 shows the modulation characteristics of the external modulation unit 51. The vertical axis represents the optical output power of the external modulation unit 51, and the horizontal axis represents the bias voltage. The figure shows an operation characteristic curve and the waveforms of the pilot superimposed signal and the output light of the external modulation unit 51.
A middle point between the maximum value and the minimum value on the operation characteristic curve of the external modulation unit 51 is the operating point of the external modulation unit 51. The figure shows a state in optimum operation of the external modulation unit 51, without the frequency component of the pilot tone appearing in the electric signal converted from the output light. The bias voltage providing the optimum operating point is denoted as bias voltage Vop.
With the low-frequency pilot tone having frequency f0, output light that has been intensity-modulated by a signal having frequency 2f0 is obtained. In the figure, Vπ represents a half-wave voltage. An amount of phase shift required to switch light between ‘0’ and ‘1’ is π, and a voltage necessary for that is referred to as a half-wave voltage.
FIGS. 13 and 14 show modulation characteristics at non-optimum operating points. If the operating point varies with temperature or time in the external modulation unit 51, the operation characteristic curve slides in the course of time along the horizontal axis on the coordinate system. FIG. 13 shows an operating point slid in the positive direction, and FIG. 14 shows an operating point slid in the negative direction.
The drift of the operating point causes the pilot tone (frequency f0) to be modulated in phase when the input data signal is ‘0’ or ‘1’. With the in-phase modulation, the output light varies at frequency f0. As shown by a comparison between output lights in FIGS. 13 and 14, the phase of the output light having frequency f0 changes by π, depending on the direction of drift of the operating point.
With a positive drift of the operating point (Vop<Va, where Va is the bias voltage for the positive drift), the envelopes of the pilot superimposed signal and output light are in phase.
This can be seen in the pilot tone as well. The internally oscillated pilot tone and the pilot tone extracted from the electric signal after monitoring by the PD are in phase. In the automatic bias control process, when it is recognized from the result of phase comparison that the phases are the same, the bias control unit 54 lowers the current bias voltage Va.
With a negative drift of the operating point (Vb<Vop, where Vb is the bias voltage for the negative drift), the envelopes of the pilot superimposed signal and output light are in opposite phases.
This can be seen in the pilot tone as well. The internally oscillated pilot tone and the pilot tone extracted from the electric signal after monitoring by the PD are in opposite phases. In the automatic bias control process, when it is recognized from the result of phase comparison that the phases are opposite, the bias control unit 54 increases the current bias voltage Vb.
By comparing the phases of the internally oscillated pilot tone and the returned pilot tone, the relative position of the current bias voltage can be known with reference to the optimum operating point, and the bias voltage can be optimized. If the frequency component of the pilot tone does not appear, it can be recognized that the external modulation unit 51 is operating in the optimum state.
The automatic bias control process as described above includes operations such as adding a pilot tone to the modulation signal input to the external modulation unit 51, extracting the pilot tone from the output light of the external modulation unit 51 by monitoring by the PD, and correcting the bias voltage. These operations require large circuitry.
Although most of the recent optical communication devices are controlled by microcontrollers (chips incorporating a computer system containing a microprocessor, memory, I/O (peripheral devices), and the like), the conventional automatic bias control process requires many circuit elements in addition to the microcontroller because the pilot tone is used. Those circuit elements include an oscillator for oscillating the low-frequency pilot tone, a band-pass filter for extracting the pilot tone, a superimposing circuit for adding the pilot tone to the input data signal, and their peripheral circuits. This results in large circuitry and complicated control process.